Movable barrier operators may be used to control access to areas by moving movable barriers between different positions. Various types of movable barriers can be moved in such a fashion, including vertically moving barriers such as single piece and segmented barriers as well as horizontally moving barriers such as sliding and swinging gates.
A movable barrier operator may have a motive source, such as a motor system, to produce movement of a movable barrier. More particularly, the motor system may have a wheel, pulley, or sprocket that engages a flexible driven member connected to the movable barrier and moves the driven member to adjust the position of the movable barrier. In one approach, the driven member is a long chain and the motor system has a drive gear that engages the chain. The chain is connected to the movable barrier such that rotating the drive gear moves the chain and the attached movable barrier. The movable barrier operator controls the rotation of the drive gear to move the movable barrier between a full-open position and a full-closed position. In a second approach the driven member is a belt and the motor system has a drive pulley that engages the belt. The belt is connected to the movable barrier such that rotating the drive pulley moves the belt and the attached movable barrier. As with the first approach, the movable barrier operator controls the rotation of the drive pulley to move the movable barrier between a full-open position and a full-closed position.
One shortcoming to using a flexible driven member is that when the movable barrier is at either the full-open or the full-closed position, there will be a long segment of the flexible driven member that sags downwardly due to the effect of gravity on the driven member. The appearance of the long sagging segment of the flexible driven member may be visually unappealing for certain applications. Also, the appearance of the flexible driven member sagging may be considered a sign of an improper installation. In this instance, an installer may be tempted to over-tighten the flexible driven member in order to reduce sag, which may cause undo wear on the system.
Further, the flexible drive member may gradually stretch over time such that an acceptable amount of sag at installation may increase and eventually become unacceptable to the owner of the movable barrier operator. If the flexible drive member is a chain, the chain may have hinge points that loosen over time and gradually increase in length. Similarly, belts have a tendency to increase in length over their lifetime due to stretching
Another shortcoming of prior movable barrier operators is that the sagging segment of chain allows the motor to speed up prior to having to pull the barrier. When the drive gear begins to rotate, the sagging segment will first be tensioned to remove the sag before the barrier is moved. This is due to the low amount of force needed to remove the tension. Once the sag is removed the operator will have to overcome the inertia of the barrier requiring a much higher force. This creates an impact force on the movable barrier operator which may damage the movable barrier operator, the flexible drive member, and the movable barrier.